This invention relates to the disposal by combustive destruction of troublesome substances, especially global-warming, air-polluting halogenated compounds, such as fluorocarbons, and particulate-forming matter upon oxidation, such as silane.
Fluorocarbon gases such as C.sub.2 F.sub.6 and CF.sub.4 are global-warming compounds when released into the atmosphere where they have extremely long lifetimes. These gases as well as other fluorinated gases such as NF.sub.3 and SF.sub.6 are used in the manufacture of semiconductors during the etching, modification and construction of silicon wafers, and during the cleaning of the machines used in the process. Hydrides, such as silane (SiH.sub.4) which ignites upon exposure to air, are also used in the process of making silicon wafers. The fluorinated gases and hydrides and even particulate matter are swept out of the machines with nitrogen, sometimes together and sometimes sequentially.
According to current practice, the nitrogen stream containing the troublesome gases is subjected to thermal destruction using electrical heat or gas firing. However, complete destruction of the undesired gases is achieved only with a large consumption of thermal energy. Another current technique of mixing the stream with hydrogen and effecting combustion is unsatisfactory because of the large usage of expensive hydrogen.
It is significant that these expensive and unsatisfactory disposal methods are in use even though several patents propose other procedures. For example, U.S. Pat. No. 4,627,388 burns halogenated hydrocarbon waste in a horizontal fire tube boiler requiring a refractory lined combustion chamber of substantial length to contain the flame front near adiabatic conditions. U.S. Pat. No. 4,206,711 uses a vertical combustion chamber wherein liquid waste is sprayed down from the top, while several flat flame radiation type burners in the walls of the chamber provide flames that totally surround the sprayed waste. U.S. Pat. No. 4,828,481 eliminates the large and costly equipment of the aforesaid patents by proposing a combustion chamber comprising two opposed porous plates between which combustion is carried out. A mixture of gaseous fuel, air, and waste vapor is fed through one porous plate, burned in the chamber, and the combustion products are exhausted through the other porous plate. However, the waste material must be free of particles or the inlet porous plate will become plugged. Even in the absence of particles in the waste material, there is the real danger that particles, such as soot or silica (if silane is in the waste), will form during combustion and plug the outlet porous plate. The need for a practical disposal system still exists.
In industrial practice, the gaseous stream carrying one or more halogenated compounds may also contain particulate-forming matter upon oxidation, simultaneously or sequentially. Silane which oxidizes in air to silica, and another often used hydride, arsine (ASH.sub.3) which oxidizes to a troublesome sticky oxide (As.sub.2 O.sub.3), are illustrative of particulate-forming matter commonly associated with halogenated compounds, particularly the fluorocarbons used in the semiconductor industry.
Besides the fluorinated gases of the semiconductor industry, air pollutants encountered in other industries include chlorinated hydrocarbons such as carbon tetrachloride, trichloroethylene, chlorobenzene and vinyl chloride. The refrigeration industry has long favored chlorofluorohydrocarbons as refrigerant gases but these gases are now being phased out of future use. A satisfactory system for the disposal of all these halogenated compounds is still wanting.
A principal object of this invention is to provide a simple and economic system for the combustive destruction of halogenated compounds and/or particulate-forming matter upon oxidation.
A further object is to provide an apparatus and a process that achieve substantially complete combustive destruction of troublesome substances while suppressing the formation of air pollutants, namely, nitrogen oxides (NO.sub.x), carbon monoxide (CO) and unburned hydrocarbons (UHC), that are commonly formed during combustion.
Another important object is to utilize apparatus that is simple and economic to construct and operate.
These and other features and advantages of the invention will be apparent from the description which follows.